U.S. patent application number 15/744426 was filed with the patent office on 2018-07-19 for powder of hexagonal boron nitride, process for producing same, resin composition, and resin sheet.
This patent application is currently assigned to SHOWA DENKO K.K.. The applicant listed for this patent is SHOWA DENKO K.K.. Invention is credited to Masaru FUKASAWA, Yuki OTSUKA.
Application Number | 20180201818 15/744426 |
Document ID | / |
Family ID | 58188749 |
Filed Date | 2018-07-19 |
United States Patent
Application |
20180201818 |
Kind Code |
A1 |
OTSUKA; Yuki ; et
al. |
July 19, 2018 |
POWDER OF HEXAGONAL BORON NITRIDE, PROCESS FOR PRODUCING SAME,
RESIN COMPOSITION, AND RESIN SHEET
Abstract
A hexagonal boron nitride (hBN) powder containing an aggregate
of primary particles of hBN, in which the powder has a primary
particle size of less than 10 .mu.m, a ratio of an average longer
diameter (L.sub.1) to an average thickness (d.sub.1) of the primary
particles, [L.sub.1/d.sub.1], of 5.0 or more and 20 or less, and a
BET specific surface area of less than 10 m.sup.2/g, and the powder
has one maximum peak in a range of a particle size of 45 .mu.m or
more and 150 .mu.m or less in a particle size distribution curve of
a hexagonal boron nitride powder classified to have a particle size
of 45 .mu.m or more and 106 .mu.m or less, and has a decrease rate
of the maximum peak of 10% or more and less than 40% when a
dispersion liquid obtained by dispersing the hexagonal boron
nitride powder in water is subjected to an ultrasonic treatment for
1 minute, the peak decrease rate being calculated by expression
(1), a method for producing the hBN powder, and a resin composition
and a resin sheet each containing the hBN powder.
Inventors: |
OTSUKA; Yuki; (Yokohama-shi,
Kanagawa, JP) ; FUKASAWA; Masaru; (Shiojiri-shi,
Nagano, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SHOWA DENKO K.K. |
Tokyo |
|
JP |
|
|
Assignee: |
SHOWA DENKO K.K.
Tokyo
JP
|
Family ID: |
58188749 |
Appl. No.: |
15/744426 |
Filed: |
August 19, 2016 |
PCT Filed: |
August 19, 2016 |
PCT NO: |
PCT/JP2016/074278 |
371 Date: |
January 12, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01P 2004/03 20130101;
C08K 2201/001 20130101; C01P 2006/11 20130101; C01B 21/064
20130101; C09K 5/14 20130101; C01P 2004/50 20130101; C01P 2006/32
20130101; C08K 2201/006 20130101; C08K 3/38 20130101; C08K 7/00
20130101; C01B 21/0645 20130101; C08K 2201/003 20130101; C08K
2003/385 20130101; C01P 2006/12 20130101; C08L 101/00 20130101;
C01P 2002/60 20130101; C01P 2004/61 20130101; C08K 2201/005
20130101; C08K 3/38 20130101; C08L 101/00 20130101 |
International
Class: |
C09K 5/14 20060101
C09K005/14; C01B 21/064 20060101 C01B021/064; C08K 3/38 20060101
C08K003/38; C08K 7/00 20060101 C08K007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 3, 2015 |
JP |
2015-174106 |
Claims
1. A hexagonal boron nitride powder comprising an aggregate of
primary particles of hexagonal boron nitride, wherein the powder
has a primary particle size of less than 10 .mu.m, a ratio of an
average longer diameter (L.sub.1) to an average thickness (d.sub.1)
of the primary particles, [L.sub.1/d.sub.1], of 5.0 or more and 20
or less, and a BET specific surface area of less than 10 m.sup.2/g,
and the powder has one maximum peak in a range of a particle size
of 45 .mu.m or more and 150 .mu.m or less in a particle size
distribution curve of the hexagonal boron nitride powder classified
to have a particle size of 45 .mu.m or more and 106 .mu.m or less,
and has a decrease rate of the maximum peak of 10% or more and less
than 40% when a dispersion liquid obtained by dispersing the
hexagonal boron nitride powder in water is subjected to an
ultrasonic treatment for 1 minute, the peak decrease rate being
calculated by the following expression (1): Peak decrease
rate=[(maximum peak height before treatment(a))-(maximum peak
height after treatment(b))]/(maximum peak height before
treatment(a)) (1).
2. The hexagonal boron nitride powder as claimed in claim 1, having
a BET specific surface area of 1.5 m.sup.2/g or more and 6.0
m.sup.2/g or less.
3. The hexagonal boron nitride powder as claimed in claim 1, having
a BET specific surface area of 1.5 m.sup.2/g or more and 5.0
m.sup.2/g or less.
4. The hexagonal boron nitride powder as claimed in claim 1, having
a crystallite size of 260 .ANG. or more and 1000 .ANG. or less.
5. The hexagonal boron nitride powder as claimed in claim 1, having
a bulk density of 0.50 g/cm.sup.3 or more.
6. A resin composition comprising 10% by volume or more and 90% by
volume or less of the hexagonal boron nitride powder as claimed in
claim 1.
7. A resin sheet comprising the resin composition as claimed in
claim 6 or a cured product thereof.
8. A method for producing the hexagonal boron nitride powder as
claimed in claim 1, the method comprising a step of mixing 100
parts by mass of a mixed powder comprising 50% by mass or more and
90% by mass or less of a boron nitride fine powder (A) and 10% by
mass or more and 50% by mass or less of a boron compound (B)
represented by a formula (B.sub.2O.sub.3).(H.sub.2O).sub.X wherein
X=0 to 3, and 1.0 part by mass or more and 15 parts by mass or less
in terms of carbon of a carbon source (C), molding a resultant
mixture, and then firing a resultant under an atmosphere comprising
a nitrogen gas, wherein the boron nitride fine powder (A) has a
ratio of an average longer diameter (L.sub.2) to an average
thickness (d.sub.2) of primary particles thereof,
[L.sub.2/d.sub.2], of 2.0 or more and 15 or less, a 50% volume
cumulative particle size D.sub.50 of 0.20 .mu.m or more and 5.0
.mu.m or less, a BET specific surface area of 5.0 m.sup.2/g or more
and 30 m.sup.2/g or less, and a crystallite size of 150 .ANG. or
more and 400 .ANG. or less.
9. The method for producing the hexagonal boron nitride powder as
claimed in claim 8, wherein the boron nitride fine powder (A) has a
50% volume cumulative particle size D.sub.50 of 0.20 .mu.m or more
and 1.0 .mu.m or less.
10. The method for producing the hexagonal boron nitride powder as
claimed in claim 8, wherein the boron nitride fine powder (A) has a
BET specific surface area of 5.0 m.sup.2/g or more and 20 m.sup.2/g
or less.
11. The method for producing the hexagonal boron nitride powder as
claimed in claim 8, wherein the boron nitride fine powder (A) has a
crystallite size of 200 .ANG. or more and 400 .ANG. or less.
12. The method for producing the hexagonal boron nitride powder as
claimed in claim 8, the method comprising a step of mixing 100
parts by mass of the mixed powder comprising 50% by mass or more
and 90% by mass or less of the boron nitride fine powder (A) and
10% by mass or more and 50% by mass or less of the boron compound
(B) represented by the formula (B.sub.2O.sub.3).(H.sub.2O).sub.X
wherein X=0 to 3, and 1.0 part by mass or more and 15 parts by mass
or less in terms of carbon of the carbon source (C), molding the
resultant mixture, and then firing the resultant under an
atmosphere comprising a nitrogen gas, wherein the boron nitride
fine powder (A) has a ratio of the average longer diameter
(L.sub.2) to the average thickness (d.sub.2) of the primary
particles thereof, [L.sub.2/d.sub.2], of 2.0 or more and 15 or
less, a 50% volume cumulative particle size D.sub.50 of 0.20 .mu.m
or more and 1.0 .mu.m or less, a BET specific surface area of 5.0
m.sup.2/g or more and 20 m.sup.2/g or less, and a crystallite size
of 200 .ANG. or more and 400 .ANG. or less.
13. The method for producing the hexagonal boron nitride powder as
claimed in claim 8, wherein the carbon source (C) is one or two
selected from the group consisting of graphite and boron carbide.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hexagonal boron nitride
(hereinafter, also simply referred to as "hBN") powder, and a resin
sheet comprising the hBN powder and particularly relates to a
high-purity hBN powder comprising an aggregate comprising primary
particles of hBN (hereinafter, also simply referred to as
"aggregate"), a method for producing the hBM powder, and a resin
composition and a resin sheet each comprising the hBN powder.
BACKGROUND ART
[0002] An hBN particle has a layered structure similar to that of
graphite, has excellent properties such as thermal conductive
properties, electric insulation, chemical stability, lubricating
properties as a solid, and thermal shock resistance, and therefore
is used as an insulation/heat dissipation material, a solid
lubricant, solid mold release agent, a raw material for producing
an hBN sintered body, and the like taking advantage of these
properties.
[0003] Conventionally, the hBN powder has generally been obtained
by mixing a boron compound such as boric acid or borax and a
nitrogen compound such as melamine or urea, then firing the
resultant mixture at a relatively low temperature under an ammonia
atmosphere or a non-oxidizing gas atmosphere to produce a crude hBN
powder having a low crystallinity, and subsequently firing the
obtained crude hBN powder at a high temperature under a
non-oxidizing gas atmosphere to allow the crystals to grow (PTLs 1
to 3).
[0004] A sheet, tape, grease, or the like in which such an hBN
powder is contained as a filler in a resin material such as an
epoxy resin, silicone rubber, or the like is used as a thermally
conductive member, such as, for example, a thermally conductive
sheet or thermally conductive grease having electric insulation,
for effectively dissipating heat generated from an electronic
component. To further improve the thermal conductive properties of
these thermally conductive members, attempts to increase the
filling rate of the hBN powder in the thermally conductive members
are being made.
[0005] However, the primary particle of hBN generally has a
scale-like particle shape, and the ratio of the average longer
diameter to the average thickness of the primary particles is high,
and therefore when the filling rate is increased, the primary
particles easily face in a constant direction and the anisotropy
easily occurs in properties of a molded article, such as a
thermally conductive sheet, obtained by molding a resin composition
comprising the hBN powder. When such anisotropy occurs, the
properties such as the thermal conductive properties, the electric
insulation, and the thermal heat resistance are lowered.
[0006] Therefore, in recent years, a method for mixing the hBN
powder comprising secondary particles (aggregate) in which primary
particles of hBN aggregate with a resin has been used for the
purpose of improving the filling rate of the hBN powder and
suppressing anisotropy in a thermally conductive sheet (PTLs 4,
5).
[0007] However, when the strength of the aggregate is not
sufficient, there is a problem that the aggregate disintegrates in
a process of forming a composite with the resin, so that the
anisotropy occurs in the thermally conductive sheet and the filling
rate of the hBN powder in the thermally conductive sheet cannot be
increased sufficiently because the disintegration of the aggregate
needs to be prevented, thereby lowering the thermal conductive
properties.
[0008] Thus, attempts to obtain the hBN powder by subjecting boron
carbide to nitriding treatment under a condition of 1800.degree. C.
or more in a nitrogen atmosphere, then mixing a resultant product
with diboron trioxide or a precursor thereof, thereafter firing the
resultant mixture, and removing a carbon component after that have
been made for the purpose of improving the filling rate of the hBN
powder in a thermally conductive sheet and improving the thermal
conductive properties (PTLs 6, 7).
[0009] However, the reaction rate for forming boron nitride from
boron carbide is very slow, and therefore there is a problem that
it requires a long time in a method involving reacting only boron
carbide with nitrogen, thereby increasing production cost.
Moreover, the thermal conductive properties of the hBN powder
obtained by the production method are still insufficient and
therefore further improvement has been desired.
CITATION LIST
Patent Literature
[0010] PTL1: JP 61-286207 A
[0011] PTL2: JP 3461651 B
[0012] PTL3: JP 5-85482 B
[0013] PTL4: JP 2011-098882 A
[0014] PTL5: JP 2005-343728 A
[0015] PTL6: JP 4750220 B
[0016] PTL7: JP 5081488 B
SUMMARY OF INVENTION
Technical Problem
[0017] The present invention intends to provide a high-purity hBN
powder comprising an aggregate comprising primary particles of hBN,
the hBN powder having a more suppressed anisotropy than
conventional hBN powders and having superior thermal conductive
properties, a method for producing the hBN powder, and a resin
composition and a resin sheet each comprising the hBN powder.
Solution to Problem
[0018] The present inventors have conducted diligent studies to
find that the problems can be solved by an hBN powder comprising an
aggregate comprising primary particles of hBN, the hBN powder
having a primary particle size, a ratio of an average longer
diameter (L.sub.1) to an average thickness (d.sub.1) of the primary
particles, [L.sub.1/d.sub.1] (hereinafter, also simply referred to
as "ratio [L.sub.1/d.sub.1]"), and a BET specific surface area in a
particular range, and having a strength of the aggregate in a
particular range. Moreover, the present inventors have found that
the strength of the aggregate can be adjusted by focusing on a
decrease rate of a maximum peak when a dispersion liquid obtained
by dispersing in water a hexagonal boron nitride powder having one
maximum peak in a particular range in a particle size distribution
curve of the hBN powder is subjected to an ultrasonic treatment for
1 minute.
[0019] The present invention is based on the above-described
findings.
[0020] That is, the present invention provides the following [1] to
[13].
[1] A hexagonal boron nitride powder comprising an aggregate of
primary particles of hexagonal boron nitride, wherein the powder
has a primary particle size of less than 10 .mu.m, a ratio of an
average longer diameter (L.sub.1) to an average thickness (d.sub.1)
of the primary particles, [L.sub.1/d.sub.1], of 5.0 or more and 20
or less, and a BET specific surface area of less than 10 m.sup.2/g,
and
[0021] the powder has one maximum peak in a range of a particle
size of 45 .mu.m or more and 150 .mu.m or less in a particle size
distribution curve of the hexagonal boron nitride powder classified
to have a particle size of 45 .mu.m or more and 106 .mu.m or less,
and has a decrease rate of the maximum peak of 10% or more and less
than 40% when a dispersion liquid obtained by dispersing the
hexagonal boron nitride powder in water is subjected to an
ultrasonic treatment for 1 minute, the peak decrease rate being
calculated by the following expression (1):
Peak decrease rate=[(maximum peak height before
treatment(a))-(maximum peak height after treatment(b))]/(maximum
peak height before treatment(a)) (1).
[2] The hexagonal boron nitride powder according to [1], having a
BET specific surface area of 1.5 m.sup.2/g or more and 6.0
m.sup.2/g or less. [3] The hexagonal boron nitride powder according
to [1] or [2], having a BET specific surface area of 1.5 m.sup.2/g
or more and 5.0 m.sup.2/g or less. [4] The hexagonal boron nitride
powder according to any one of [1] to [3], having a crystallite
size of 260 .ANG. or more and 1000 .ANG. or less. [5] The hexagonal
boron nitride powder according to any one of [1] to [4], having a
bulk density of 0.50 g/cm.sup.3 or more. [6] A resin composition
comprising 10% by volume or more and 90% by volume or less of the
hexagonal boron nitride powder according to any one of [1] to [5].
[7] A resin sheet comprising the resin composition according to [6]
or a cured product thereof. [8] A method for producing the
hexagonal boron nitride powder according to any one of [1] to [5],
the method comprising a step of mixing 100 parts by mass of a mixed
powder comprising 50% by mass or more and 90% by mass or less of a
boron nitride fine powder (A) and 10% by mass or more and 50% by
mass or less of a boron compound (B) represented by a formula
(B.sub.2O.sub.3).(H.sub.2O).sub.X wherein X=0 to 3, 1.0 part by
mass or more and 15 parts by mass or less in terms of carbon of a
carbon source (C), molding a resultant mixture, and then firing a
resultant mixture under an atmosphere comprising a nitrogen gas,
wherein
[0022] the boron nitride fine powder (A) has a ratio of an average
longer diameter (L.sub.2) to an average thickness (d.sub.2) of
primary particles thereof, [L.sub.2/d.sub.2], of 2.0 or more and 15
or less, a 50% volume cumulative particle size D.sub.50 of 0.20
.mu.m or more and 5.0 .mu.m or less, a BET specific surface area of
5.0 m.sup.2/g or more and 30 m.sup.2/g or less, and a crystallite
size of 150 .ANG. or more and 400 .ANG. or less.
[9] The method for producing the hexagonal boron nitride powder
according to [8], wherein the boron nitride fine powder (A) has a
50% volume cumulative particle size D.sub.50 of 0.20 .mu.m or more
and 1.0 .mu.m or less. [10] The method for producing the hexagonal
boron nitride powder according to [8] or [9], wherein the boron
nitride fine powder (A) has a BET specific surface area of 5.0
m.sup.2/g or more and 20 m.sup.2/g or less. [11] The method for
producing the hexagonal boron nitride powder according to any one
of [8] to [10], wherein the boron nitride fine powder (A) has a
crystallite size of 200 .ANG. or more and 400 .ANG. or less. [12]
The method for producing the hexagonal boron nitride powder
according to any one of [8] to [11], the method comprising a step
of mixing 100 parts by mass of the mixed powder comprising 50% by
mass or more and 90% by mass or less of the boron nitride fine
powder (A) and 10% by mass or more and 50% by mass or less of the
boron compound (B) represented by the formula
(B.sub.2O.sub.3).(H.sub.2O).sub.X wherein X=0 to 3, and 1.0 part by
mass or more and 15 parts by mass or less in terms of carbon of the
carbon source (C), molding the resultant mixture, and then firing
the resultant under an atmosphere comprising a nitrogen gas,
wherein
[0023] the boron nitride fine powder (A) has a ratio of the average
longer diameter (L.sub.2) to the average thickness (d.sub.2) of
primary particles thereof, [L.sub.2/d.sub.2], of 2.0 or more and 15
or less, a 50% volume cumulative particle size D.sub.50 of 0.20
.mu.m or more and 1.0 .mu.m or less, a BET specific surface area of
5.0 m.sup.2/g or more and 20 m.sup.2/g or less, and a crystallite
size of 200 .ANG. or more and 400 .ANG. or less.
[13] The method for producing the hexagonal boron nitride powder
according to any one of [8] and [12], wherein the carbon source (C)
is at least one selected from the group consisting of graphite and
boron carbide.
Advantageous Effects of Invention
[0024] According to the present invention, a high-purity hBN powder
comprising an aggregate comprising primary particles of hBN, the
hBN powder having a more suppressed anisotropy than conventional
hBN powders and having superior thermal conductive properties, a
method for producing the hBN powder, and a resin composition and a
resin sheet each comprising the hBN powder can be provided.
BRIEF DESCRIPTION OF DRAWINGS
[0025] FIG. 1 is a schematic diagram of an aggregate of primary
particles of hBN according to the present invention.
[0026] FIG. 2 is an SEM image of an aggregate of primary particles
of hBN obtained in Example 1.
[0027] FIG. 3 is an enlarged SEM image of an aggregate of primary
particles of hBN obtained in Example 1.
[0028] FIG. 4 is an enlarged SEM image of an aggregate of primary
particles of hBN obtained in Comparative Example 2.
[0029] FIG. 5 is a schematic diagram of a resin sheet comprising a
hexagonal boron nitride powder according to the present
invention.
[0030] FIG. 6 is a graph showing particle size distribution curves
before and after an ultrasonic treatment of Example 1.
DESCRIPTION OF EMBODIMENTS
[Hexagonal Boron Nitride Powder]
[0031] The hexagonal boron nitride powder according to the present
invention is
[0032] a hexagonal boron nitride powder comprising an aggregate of
primary particles of hexagonal boron nitride, wherein
[0033] the powder has a primary particle size of less than 10
.mu.m, a ratio, [L.sub.1/d.sub.1], of the primary particles of 5.0
or more and 20 or less, and a BET specific surface area of less
than 10 m.sup.2/g, and
[0034] the powder has one maximum peak in a range of a particle
size of 45 .mu.m or more and 150 .mu.m or less in a particle size
distribution curve of the hexagonal boron nitride powder classified
to have a particle size of 45 .mu.m or more and 106 .mu.m or less,
and has a decrease rate of the maximum peak of 10% or more and less
than 40% when a dispersion liquid obtained by dispersing the
hexagonal boron nitride powder in water is subjected to an
ultrasonic treatment for 1 minute, the peak decrease rate being
calculated by the following expression (1).
Peak decrease rate=[(maximum peak height before
treatment(a))-(maximum peak height after treatment(b))]/(maximum
peak height before treatment(a)) (1)
[0035] According to the present invention, a high-purity hBN powder
comprising an aggregate comprising primary particles of hBN, the
hBN powder having a more suppressed anisotropy than conventional
hBN powders and having superior thermal conductive properties can
be obtained. The reason that such an effect is obtained is not
clear but is considered as follows.
[0036] The hBN powder according to the present invention comprises
a dense aggregate because the primary particles of hBN which form
the aggregate have a particle size in a particular range and have a
ratio [L1/d1] and a BET specific surface area each in a particular
range, and the hBN powder according to the present invention also
comprises a strong aggregate because the strength of the aggregate
measured under a particular condition is in a particular range.
Therefore, the aggregate can maintain a granular shape without
disintegrating in the process of forming a composite with a resin,
and the filling rate of the hBN powder in a resin composition can
be improved. It is considered that as a result, high thermal
conductive properties can be exhibited.
[0037] Moreover, it is inferred that facing of the primary
particles in a constant direction due to the disintegration of the
aggregate can be suppressed and the anisotropy can be suppressed
because the hBN powder according to the present invention comprises
a dense and strong aggregate.
[0038] However, these are estimates, and the present invention is
not limited to these mechanisms.
<Primary Particles>
[0039] The primary particle size of the hBN powder according to the
present invention is less than 10 .mu.m, preferably 0.50 .mu.m or
more and less than 10 .mu.m, more preferably 1.0 .mu.m or more and
8.0 .mu.m or less, still more preferably 1.0 .mu.m or more and 6.0
.mu.m or less, further still more preferably 1.5 .mu.m or more and
5.0 .mu.m or less, further still more preferably 2.0 .mu.m or more
and 5.0 .mu.m or less, and further still more preferably 2.0 .mu.m
or more and 4.0 .mu.m or less in average from the viewpoint of
improvements in the thermal conductive properties. In the hBN
powder comprising a dense aggregate comprising small primary
particles having a primary particle size of less than 10 .mu.m, the
aggregate can maintain the granular shape without disintegrating in
the process of forming a composite with a resin, and the filling
rate in a resin composition can be improved. Therefore, the thermal
conductive properties can be improved.
[0040] It is to be noted that the primary particle size is a
numerical average value of longer diameters of the primary
particles and is measured by the method described in Examples.
[0041] The primary particles contained in the hBN powder according
to the present invention are scale-like. The "scale-like" herein
means a shape having a ratio of an average longer diameter
(L.sub.1) to an average thickness (d.sub.1), [L.sub.1/d.sub.1], for
the primary particles of 5.0 or more and 20 or less. Even in the
case where the primary particles are scale-like in this way, the
primary particles form an aggregate, and therefore the filling rate
of the hBN powder in a resin composition can be improved. Moreover,
the strength of the aggregate is high, and therefore the
disintegration of the aggregate can be suppressed, and orientation
of the primary particles in a constant direction can be prevented
or suppressed, so that the anisotropy can be suppressed.
[0042] In the present specification, the "average longer diameter"
means a number average value of the longer diameters of the primary
particles, and the "average thickness" means a number average value
of the thicknesses of the primary particles. In addition, the
"longer diameter" means the maximum diameter in a planar direction
of a scale-like particle.
[0043] The ratio [L.sub.1/d.sub.1] for the primary particles in the
hBN powder according to the present invention is 5.0 or more and 20
or less, preferably 7.0 or more and 18 or less, more preferably 9.0
or more and 17 or less, still more preferably 9.5 or more and 16 or
less, further still more preferably 10 or more and 15 or less, and
further still more preferably 10.5 or more and 14.5 or less from
the viewpoint of suppressing the anisotropy and improving the
thermal conductive properties.
[0044] It is to be noted that the ratio [L.sub.1/d.sub.1] for the
primary particles contained in the hBN powder is measured by the
method described in Examples.
<hBN Powder>
[0045] The BET specific surface area of the hBN powder according to
the present invention is less than 10 m.sup.2/g, preferably 1.0
m.sup.2/g or more and 9.5 m.sup.2/g or less, more preferably 1.5
m.sup.2/g or more and 9.0 m.sup.2/g or less, still more preferably
1.5 m.sup.2/g or more and 8.0 m.sup.2/g or less, further still more
preferably 2.0 m.sup.2/g or more and 7.0 m.sup.2/g or less, further
still more preferably 2.5 m.sup.2/g or more and 6.0 m.sup.2/g or
less, further still more preferably 3.0 m.sup.2/g or more and 5.0
m.sup.2/g or less, and further still more preferably 3.5 m.sup.2/g
or more and 4.5 m.sup.2/g or less from the viewpoint of
improvements in the thermal conductive properties. When the BET
specific surface area is less than 10 m.sup.2/g, the specific
surface area of the aggregate contained in the hBN powder is also
small and the amount of a resin component to be taken in the
aggregate in producing a resin composition is small. Therefore, it
is considered that the thermal conductive properties are improved
because the amount of the resin component existing between the
aggregates becomes relatively large to improve the dispersibility
of the aggregates to the resin component, so that the hBN powder
and the resin component become well blended.
[0046] It is to be noted that the BET specific surface area of the
hBN powder is measured by the BET one-point method utilizing the
fluid process described in Examples.
[0047] The hBN powder according to the present invention has one
maximum peak in a range of a particle size of 45 .mu.m or more and
150 .mu.m or less in a particle size distribution curve of the hBN
powder classified to have a particle size of 45 .mu.m or more and
106 .mu.m or less, and has a decrease rate of the maximum peak of
10% or more and less than 40% when a dispersion liquid obtained by
dispersing the hBN powder in water is subjected to an ultrasonic
treatment for 1 minute, the decrease rate being calculated by the
following expression (1):
Peak decrease rate=[(maximum peak height before
treatment(a))-(maximum peak height after treatment(b))]/(maximum
peak height before treatment(a)) (1).
[0048] The particle size distribution curve is measured using a
particle size distribution analyzer by the laser diffraction
scattering method. The lower the peak decrease rate is, the higher
the disintegration strength of the hBN powder is, and therefore the
peak decrease rate is an index of the disintegration strength of
the hBN powder. Accordingly, the disintegration of the aggregate in
the process of forming a composite with a resin can be prevented or
suppressed by setting the peak decrease rate to less than 40%. In
addition, the insulation is improved by setting the peak decrease
rate to 10% or more. Further, in the case where the hBN powder is
used as a resin sheet obtained by molding a resin composition, the
moldability is improved and the aggregate deforms moderately in the
resin sheet by setting the peak decrease rate to 10% or more, and
thereby the contact property of the hBN powder being a filler is
improved to form a thermal conduction path, so that high thermal
conduction properties can be exhibited. From these viewpoints, the
peak decrease rate of the hBN powder is 10% or more and less than
40%, preferably 15% or more and 38% or less, more preferably 20% or
more and 35% or less, still more preferably 23% or more and 30% or
less, and further still more preferably 23% or more and 28% or
less.
[0049] It is to be note that the peak decrease rate of the hBN
powder is measured by the method described in Examples.
[0050] In addition, by the hBN powder "classified to have a
diameter of 45 .mu.m or more and 106 .mu.m or less" in the present
invention, a pre-treatment condition of the hBN powder according to
the present invention provided for the measurement of the peak
decrease rate is specified, but the hBN powder itself according to
the present invention is not specified.
[0051] The crystallite size of the hBN powder according to the
present invention is preferably 260 .ANG. or more and 1000 .ANG. or
less, more preferably 280 .ANG. or more and 750 .ANG. or less,
still more preferably 300 .ANG. or more and 500 .ANG. or less, and
further still more preferably 320 .ANG. or more and 400 .ANG. or
less from the viewpoint of improvements in the thermal conductive
properties and of suppressing the anisotropy of the thermal
conductivity. When the crystallite size is 260 .ANG. or more, the
inconsistency of the crystallites can be suppressed, so that high
thermal conductive properties are exhibited. In addition, when the
crystallite size is 1000 .ANG. or less, the anisotropy of the
thermal conductive properties can be suppressed.
[0052] It is to be noted that the crystallite size is measured by
the method described in Examples.
[0053] The bulk density of the hBN powder according to the present
invention is preferably 0.50 g/cm.sup.3 or more, more preferably
0.60 g/cm.sup.3 or more, still more preferably 0.70 g/cm.sup.3,
further still more preferably 0.75 g/cm.sup.3 or more, and further
still more preferably 0.80 g/cm.sup.3 or more from the viewpoint of
improvements in the strength of the aggregate.
[0054] It is to be noted that the bulk density of the hBN powder is
measured by the method described in Examples.
[0055] The 50% volume cumulative particle size (D.sub.50) of the
hBN powder according to the present invention is preferably 10
.mu.m or more and 150 .mu.m or less, more preferably 15 .mu.m or
more and 100 .mu.m or less, still more preferably 20 .mu.m or more
and 70 .mu.m or less, further still more preferably 30 .mu.m or
more and 50 .mu.m or less, and further still more preferably 35
.mu.m or more and 45 .mu.m or less from the viewpoint of
improvements in the strength of the aggregate and in the filling
rate.
[0056] It is to be noted that the 50% volume cumulative particle
size (D.sub.50) of the hBN powder is measured by the method
described in Examples.
[0057] The hBN powder according to the present invention has a
content of a powder not passing through a sieve having an opening
of 45 .mu.m determined using a reduced pressure suction type
sieving machine (air jet sieve) of preferably 50% by mass or more,
more preferably 60% by mass or more, still more preferably 70% by
mass or more, further still more preferably 75% by mass or more,
and further still more preferably 80% by mass or more from the
viewpoint of improvements in the thermal conductive properties, and
is preferably 90% by mass or less, more preferably 85% by mass or
more from the viewpoint of easiness of production.
[0058] It is to be noted that the content of the powder not passing
through the sieve having an opening of 45 .mu.m is measured by the
method described in Examples.
[0059] The hBN powder according to the present invention comprises
the aggregate, and therefore the aggregate can maintain the
granular shape without disintegrating and the orientation of the
primary particles in a constant direction is prevented or
suppressed even though the filling rate of the hBN powder in a
resin composition is increased. Therefore, a resin composition and
a resin sheet each having a suppressed anisotropy and having
excellent thermal conductive properties can be obtained by using
the hBN powder.
[0060] The purity of the hBN powder according to the present
invention, namely the purity of hBN in the hBN powder according to
the present invention is preferably 96% by mass or more, more
preferably 98% by mass or more, still more preferably 99% by mass
or more, further still more preferably 99.5% by mass or more, and
further still more preferably 99.8% by mass or more from the
viewpoint of improvements in the thermal conductive properties.
[0061] It is to be noted that the purity of the hBN powder can be
measured by the method described in Examples.
[0062] The content of boron oxide (hereinafter, also simply
referred to as "B.sub.2O.sub.3 content") in the hBN powder
according to the present invention is preferably 0.001% by mass or
more and 0.120% by mass or less, more preferably 0.005% by mass or
more and 0.110% by mass or less, still more preferably 0.008% by
mass or more and 0.100% by mass or less, further still more
preferably 0.010% by mass or more and 0.080% by mass or less, and
further still more preferably 0.020% by mass or more and 0.070% by
mass or less from the viewpoint of improvements in the thermal
conductive properties and production superiority.
[0063] It is to be noted that the B.sub.2O.sub.3 content can be
measured by the method described in Examples.
[0064] The content of calcium oxide (hereinafter, also simply
referred to as "CaO") in the hBN powder according to the present
invention is preferably 0.50% by mass or less, more preferably
0.20% by mass or less, still more preferably 0.10% by mass or less,
further still more preferably 0.05% by mass or less, further still
more preferably 0.04% by mass or less, further still more
preferably 0.03% by mass or less, and further still more preferably
0.02% by mass or less from the viewpoint of improvements in the
thermal conductive properties.
[0065] It is to be noted that the content of CaO in the hBN powder
can be measured by the method described in Examples.
[0066] The content of carbon in the hBN powder according to the
present invention is preferably 0.50% by mass or less, more
preferably 0.20% by mass or less, still more preferably 0.10% by
mass or less, further still more preferably 0.05% by mass or less,
further still more preferably 0.04% by mass or less, further still
more preferably 0.03% by mass or less, and further still more
preferably 0.02% by mass or less from the viewpoint of improvements
in the thermal conductive properties and the electric
insulation.
[0067] It is to be noted that the content of carbon in the hBN
powder can be measured by the method described in Examples.
<Surface Treatment>
[0068] A surface treatment may be performed as necessary on the hBN
powder according to the present invention using various coupling
agents or the like for the purpose of enhancing the dispersibility
in the resin component and improving the processability in
producing a resin composition by dispersing the hBN powder
according to the present invention in a resin component.
(Coupling Agent)
[0069] Examples of the coupling agent include silane-based,
titanate-based, and aluminum-based coupling agents, and among
these, silane-based coupling agents are preferable in terms of the
effect. As the silane-based coupling agent, aminosilane compounds
such as .gamma.-aminopropyltrimethoxysilane,
.gamma.-aminopropyltriethoxysilane,
.gamma.-(2-aminoethyl)aminopropyltrimethoxysilane,
.gamma.-(2-aminoethyl)aminopropyltriethoxysilane,
.gamma.-anilinopropyltrimethoxysilane,
.gamma.-anilinopropyltriethoxysilane,
N-.beta.-(N-vinylbenzylaminoethyl)-.gamma.-aminopropyltrimethoxysilane,
and
N-.beta.-(N-vinylbenzylaminoethyl)-.gamma.-aminopropyltriethoxysilane
are particularly preferably used.
[Method for Producing Hexagonal Boron Nitride Powder]
[0070] The hexagonal boron nitride powder (hBN powder) according to
the present invention is preferably obtained by a production method
comprising a step of mixing 100 parts by mass of a mixed powder
comprising 50% by mass or more and 90% by mass or less of a boron
nitride fine powder (A) (hereinafter, also simply referred to as
"BN fine powder (A)") and 10% by mass or more and 50% by mass or
less of an oxygen-containing boron compound (B) represented by a
formula (B.sub.2O.sub.3) (H.sub.2O).sub.X wherein X=0 to 3
(hereinafter, also simply referred to as "boron compound (B)", and
1.0 part by mass or more and 15 parts by mass or less in terms of
carbon of a carbon source (C), molding a resultant mixture, and
then firing a resultant under an atmosphere comprising a nitrogen
gas from the viewpoint of suppressing the anisotropy and the
viewpoint of improvements in the thermal conductive properties.
[0071] It is to be noted that with respect to the hBN powder
according to the present invention, the hBN powder is preferably
obtained by further performing at least one of pulverization or
classification after the firing step, and the hBN powder is more
preferably obtained by performing both the pulverization and the
classification.
[0072] Hereinafter, the boron nitride fine powder (BN fine powder)
(A), the boron compound (B), and the carbon source (C) will be
described, and subsequently each step of mixing, molding, firing,
pulverization, and classification will be described.
<Mixed Powder>
[0073] The mixed powder for use in the production method according
to the present invention comprises 50% by mass or more and 90% by
mass or less of the BN fine powder (A) and 10% by mass or more and
50% by mass or less of the boron compound (B). The BN fine powder
(A) has a ratio of an average longer diameter (L.sub.2) to an
average thickness (d.sub.2) of primary particles thereof,
[L.sub.2/d.sub.2] (hereinafter, also simply referred to as "ratio
[L.sub.2/d.sub.2]"), of 2.0 or more and 15 or less, a 50% volume
cumulative particle size D.sub.50 of 0.20 .mu.m or more and 5.0
.mu.m or less, a BET specific surface area of 5.0 m.sup.2/g or more
and 30 m.sup.2/g or less, and a crystallite size of 150 .ANG. or
more and 400 .ANG. or less.
[0074] When the BN fine powder (A) is contained, the BN particles
densely exist in a molded body obtained from the BN fine powder
(A), and therefore the particles grow in a state where the BN
particles are entangled during firing, so that the hBN powder
comprising a dense and strong aggregate can be obtained. Thereby,
the anisotropy of the primary particles of hBN in the resin
composition and the resin sheet described later can be suppressed,
and the thermal conductive properties of the resin composition and
the resin sheet can be improved.
[0075] The mixed powder is preferably obtained by mixing the BN
fine powder (A) and the boron compound (B) so that the content of
the BN fine powder (A) can be 50% by mass or more and 90% by mass
or less and the content of the boron compound (B) can be 10% by
mass or more and 50% by mass or less.
[Boron Nitride Fine Powder (A)]
[0076] The BN fine powder (A) has a hexagonal crystal structure,
and the ratio [L.sub.2/d.sub.2] in the primary particles of the BN
fine powder (A) is 2.0 or more and 15 or less, preferably 5.0 or
more and 10 or less, more preferably 5.0 or more and 8.0 or less,
and still more preferably 5.0 or more and 7.0 or less from the
viewpoint of suppressing the anisotropy of the primary particles of
hBN in the resin composition and the resin sheet and the viewpoint
of improvements in the thermal conductive properties of the resin
composition and the resin sheet.
[0077] The 50% volume cumulative particle size D.sub.50 of the BN
fine powder (A) is 0.20 .mu.m or more and 5.00 .mu.m or less,
preferably 0.20 .mu.m or more and 4.00 .mu.m or less, more
preferably 0.20 .mu.m or more and 3.00 .mu.m or less, still more
preferably 0.20 .mu.m or more and 2.00 .mu.m or less, further still
more preferably 0.20 .mu.m or more and 1.00 .mu.m or less, further
still more preferably 0.25 .mu.m or more and 0.90 .mu.m or less,
and further still more preferably 0.30 .mu.m or more and 0.80 .mu.m
or less from the viewpoint of suppressing the anisotropy of the
primary particles of hBN in the resin composition and the resin
sheet and the viewpoint of improvements in the thermal conductive
properties of the resin composition and the resin sheet.
[0078] The BET specific surface area of the BN fine powder (A) is
5.0 m.sup.2/g or more and 30 m.sup.2/g or less, preferably 5.0
m.sup.2/g or more and 25 m.sup.2/g or less, more preferably 5.0
m.sup.2/g or more and 20 m.sup.2/g or less, still more preferably
6.0 m.sup.2/g or more and 15 m.sup.2/g or less, further still more
preferably 6.0 m.sup.2/g or more and 13 m.sup.2/g or less, further
still more preferably 7.0 m.sup.2/g or more and 12 m.sup.2/g or
less, further still more preferably 8.0 m.sup.2/g or more and 11
m.sup.2/g or less, and further still more preferably 8.0 m.sup.2/g
or more and 10 m.sup.2/g or less from the viewpoint of suppressing
the anisotropy of the primary particles of hBN in the resin
composition and the resin sheet and the viewpoint of improvements
in the thermal conductive properties of the resin composition and
the resin sheet.
[0079] The crystallite size of the BN fine powder (A) is 150 .ANG.
or more and 400 .ANG. or less, preferably 180 .ANG. or more and 400
.ANG. or less, more preferably 200 .ANG. or more and 400 .ANG. or
less, still more preferably 220 .ANG. or more and 350 .ANG. or
less, further still more preferably 230 .ANG. or more and 320 .ANG.
or less, and further still more preferably 240 .ANG. or more and
300 .ANG. or less from the viewpoint of suppressing the anisotropy
of the primary particles of hBN in the resin composition and the
resin sheet and the viewpoint of improvements in the thermal
conductive properties of the resin composition and the resin
sheet.
[0080] The purity of the BN fine powder (A) is preferably 95% by
mass or more, more preferably 97% by mass or more, and still more
preferably 99% by mass or more.
[0081] The content of the BN fine powder (A) in the mixed powder is
50% by mass or more and 90% by mass or less. When the content of
the BN fine powder (A) in the mixed powder is 50% by mass or more,
the hBN powder according to the present invention can be produced
in a highly efficient manner by using the mixed powder as a raw
material. When the content of the BN fine powder (A) is 90% by mass
or less, production can be conducted efficiently while controlling
the amount of the BN fine powder (A) to be used. From the
viewpoints, the content of the BN fine powder (A) in the mixed
powder is preferably 55% by mass or more, and more preferably 60%
by mass or more, and is preferably 85% by mass or less, more
preferably 80% by mass or less, still more preferably 75% by mass
or less, and further still more preferably 70% by mass or less.
[Boron Compound (B)]
[0082] Examples of the boron compound (B) represented by the
formula (B.sub.2O.sub.3).(H.sub.2O).sub.X wherein X=0 to 3 include
at least one selected from the group consisting of oxides of boron
including oxo acids of boron such as orthoboric acid
(H.sub.3BO.sub.3), metaboric acid (HBO.sub.2), and tetraboric acid
(H.sub.2B.sub.4O.sub.7), and boric anhydride (B.sub.2O.sub.3).
Among others, boric anhydride (B.sub.2O.sub.3) is preferable from
the viewpoint of an easy availability and a good miscibility with
the BN fine powder (A).
[0083] The purity of the boron compound (B) is preferably 90% by
mass or more, more preferably 95% by mass or ore, still more
preferably 99% by mass or more, and further still more preferably
100% by mass.
[0084] The content of the boron compound (B) in the mixed powder is
10% by mass or more and 50% by mass or less. When the content of
the boron compound (B) in the mixed powder is 10% by mass or more,
production can be conducted efficiently while controlling the
amount of the BN fine powder (A) to be used. When the content of
the boron compound (B) is 50% by mass or less, the hBN powder
according to the present invention can be produced in a highly
efficient manner. From the viewpoints, the content of the boron
compound (B) in the mixed powder is preferably 15% by mass or more,
more preferably 20% by mass or more, still more preferably 25% by
mass or more, and further still more preferably 30% by mass or more
and is preferably 45% by mass or less, and more preferably 40% by
mass or less.
[0085] It is to be noted that the total content of the BN fine
powder (A) and the boron compound (B) in the mixed powder is
preferably 90% by mass or more, more preferably 95% by mass or
more, still more preferably 99% by mass or more, and further still
more preferably 100% by mass.
[0086] The mixed powder may comprise another component within a
range that does not impair the effects of the present invention,
and the content of the another component in the mixed powder is
preferably 10% by mass or less, more preferably 5% by mass or less,
and still more preferably 1% by mass or less, and the mixed powder
further still more preferably does not comprise the another
component.
<Carbon Source (C)>
[0087] In the production method according to the present invention,
1.0 part by mass or more and 15 parts by mass or less in terms of
carbon of the carbon source (C) is mixed, based on 100 parts by
mass of the mixed powder.
[0088] The carbon source (C) for use in the production method
according to the present invention is carbon or a carbon-containing
compound. Examples of the carbon source (C) for use in the present
invention include graphite, carbon black, boron carbide,
saccharides, melamine, and phenol resins, and the carbon source is
more preferably one or two selected from the group consisting of
graphite and boron carbide. Moreover, graphite and boron carbide
may be used together from the viewpoint of the strength of the
aggregate and the viewpoint of reducing production cost.
[0089] The content of carbon in the carbon source (C) is preferably
90% by mass or more, more preferably 95% by mass or more, still
more preferably 99% by mass or more, and further still more
preferably 100% by mass.
[0090] In the case where the carbon source (C) is present at 1.0
part by mass or more in terms of carbon, the grain growth of the
primary particles is facilitated, and nitriding the boron compound
progresses to improve the crystallinity of the aggregate, and
therefore the disintegration strength of the aggregate is improved.
In the case where the carbon source (C) is present at 15 parts by
mass or less in terms of carbon, an unreacted carbon component is
prevented from being left as a foreign body, namely a black foreign
body, to improve the degree of whiteness and the electric
insulation.
[0091] From the viewpoints, the amount of the carbon source (C) to
be blended based on 100 parts by mass of the mixed powder is
preferably 1.0 parts by mass or more and 13 parts by mass or less,
more preferably 2.0 parts by mass or more and 10 parts by mass or
less, still more preferably 2.0 parts by mass or more and 8.0 parts
by mass or less, and further still more preferably 2.5 parts by
mass or more and 5.0 parts by mass or less in terms of carbon.
[0092] In the production method according to the present invention,
the carbon source (C) includes preferably boron carbide
(hereinafter, also simply referred to as "B.sub.4C") from the
viewpoint of the thermal conductive properties. The amount of boron
carbide to be blended as the carbon source (C) based on 100 parts
by mass of the mixed powder is preferably 1.0 part by mass or more
and 15 parts by mass or less, more preferably 1.0 part by mass or
more and 13 parts by mass or less, still more preferably 2.0 parts
by mass or more and 10.0 parts by mass or less, further still more
preferably 2.0 parts by mass or more and 8.0 parts by mass or less,
and further still more preferably 2.5 parts by mass or more and 5.0
parts by mass or less in terms of carbon.
[0093] Thereby, the production of hBN progresses from carbon in the
boron carbide crystal as a starting point to produce 4 mol of hBN
from 1 mol of B.sub.4C, facilitating the grain growth of the hBN
primary particles in the thickness direction and also contributing
to the production of the dense and strong aggregate, and therefore
boron carbide is advantageous for the improvements in the thermal
conductivity. Moreover, the granular shape can be maintained in the
process of forming a composite with a resin and the disintegration
of the aggregate can be prevented or suppressed.
[0094] In addition, in the case where graphite and boron carbide
are used together as the carbon source (C), the firing time becomes
short, and therefore the production cost can be reduced more, and
the number of black foreign bodies due to boron carbide can be
reduced more than in the case where boron carbide is used singly.
Furthermore, the hBN powder comprising the denser and stronger
aggregate in which the form of the boron carbide particles is
maintained can be produced more than the hBN powder which is grown
through dissolution and deposition from crystal cores produced at
the surface of graphite when graphite is used singly as the carbon
source (C).
[0095] The content of the boron carbide in the boron carbide is
preferably 90% by mass or more, preferably 95% by mass or more,
still more preferably 99% by mass or more, and further still more
preferably 100% by mass.
[0096] In addition, the mass ratio of boron carbide to graphite
(boron carbide/graphite) in the case where graphite and boron
carbide are used together is preferably 5/95 to 50/50, more
preferably 10/90 to 40/60, and still more preferably 20/80 to 35/65
in terms of carbon.
(Mixing)
[0097] The production method according to the present invention
comprises a mixing step of mixing the BN fine powder (A) and the
boron compound (B) to obtain a mixed powder firstly, and then
mixing the mixed powder and the carbon source (C) to obtain a
mixture. The method of mixing these is not particularly limited,
and any of wet mixing and dry mixing may be used, but the wet
mixing is preferable. The wet mixing can be performed using a
general mixer such as a Henschel mixer, a ball mill, or a ribbon
blender.
[0098] In addition, a binder may be added and mixed in the mixing.
The binder is not particularly limited, examples thereof include
resins such as polyvinyl alcohol (PVA), cellulose, and
polyvinylidene fluoride (PVDF), and polyvinyl alcohol is preferably
used.
[0099] The binder is preferably used as an aqueous binder solution
obtained by dissolving these resins in water. The resin content in
the aqueous binder solution is preferably 1% by mass or more and
15% by mass or less, more preferably 1% by mass or more and 10% by
mass or less, and still more preferably 1% by mass or more and 5%
by mass or less. The amount of the aqueous binder solution to be
mixed, namely to be blended, based on 100 parts by mass of the
mixed powder is preferably 1 part by mass or more and 20 parts by
mass or less, more preferably 5 parts by mass or more and 15 parts
by mass or less, and still more preferably 8 parts by mass or more
and 12 parts by mass or less.
(Molding)
[0100] The production method according to the present invention
comprises a step of subsequently molding the mixture obtained
through the mixing into an appropriate shape to obtain a molded
body. The shape is not particularly limited; however, a columnar
shape such as a tablet is preferable from the viewpoint of ease of
handling.
[0101] The molding is performed so that the density of the molded
body can be preferably 0.50 g/cm.sup.3 or more, more preferably
0.80 g/cm.sup.3 or more, still more preferably 1.0 g/cm.sup.3 or
more and can be preferably 2.0 g/cm.sup.3 or less, more preferably
1.8 g/cm.sup.3 or less, and still more preferably 1.5 g/cm.sup.3 or
less from the viewpoint of improvements in the strength of the
aggregate in which the primary particles of hBN aggregate, and of
productivity, good handling, and reactivity.
[0102] It is to be noted that "the density of the molded body"
means the density of the molded body before drying in the case
where the mixing is the wet mixing, and in the case where a binder
and water are used in the mixing, "the density of the molded body"
means the density of the molded body including the binder and
water.
(Firing)
[0103] The production method according to the present invention
comprises a step of firing the molded body obtained through the
molding. By subjecting the mixture to press molding to make the
molded body and then firing the molded body, the boron compound (B)
contained in the molded body reacts with carbon contained in the
carbon source (C) to produce the hBN aggregate having a high
disintegration strength, and the hBN powder according to the
present invention is obtained. It is to be noted that in the case
where the firing is performed without performing the molding, it is
difficult to produce the hBN aggregate having a high disintegration
strength sufficiently.
[0104] The atmosphere during the firing is an atmosphere comprising
a nitrogen gas. The nitrogen gas concentration in the atmosphere
comprising the nitrogen gas is preferably 60% by volume or more,
more preferably 80% by volume or more, still more preferably 90% by
volume or more, and further still more preferably 99% by volume or
more. With respect to an oxygen gas, the less, the better.
[0105] The firing temperature is preferably 1000.degree. C. or more
and 2200.degree. C. or less. When the firing temperature is
1000.degree. C. or more, a sufficient reductive nitriding reaction
progresses. In addition, when the firing temperature is
2200.degree. C. or less, the occurrence of the decomposition of hBN
is prevented. From the viewpoints, the firing temperature is more
preferably 1500.degree. C. or more and 2200.degree. C. or less,
still more preferably 1600.degree. C. or more and 2200.degree. C.
or less, and further still more preferably 1700.degree. C. or more
and 2200.degree. C. or less.
[0106] The firing time is preferably 1 hour or more and 20 hours or
less. When the firing time is 1 hour or more, the reductive
nitriding reaction progresses sufficiently, and an unreacted carbon
component is prevented from being left as a black substance. In
addition, when the firing time is 20 hours or less, firing cost is
reduced. From this viewpoint, the firing time is more preferably 1
hour or more and 15 hours or less, still more preferably 3 hours or
more and 10 hours or less, further still more preferably 4 hours or
more and 9 hours or less, and further still more preferably 5 hours
or more and 7 hours or less.
[0107] It is to be noted that drying may be performed before the
firing. The drying temperature is preferably 150.degree. C. or more
and 400.degree. C. or less, more preferably 200.degree. C. or more
and 400.degree. C. or less, and the drying time is preferably 6
hours or more and 8 hours or less.
(Pulverization)
[0108] Subsequently, the fired product obtained through the firing
is preferably pulverized.
[0109] The pulverization method is not particularly limited, and
pulverization with a jaw crusher and coarse roll pulverization can
be adopted.
(Classification)
[0110] Subsequently, the pulverized product obtained through the
pulverization is preferably classified after the firing step.
[0111] The classification method is not particularly limited, and
classification can be performed with a vibrating sieve apparatus or
by air flow classification, water sieving, centrifugal separation,
or the like. Among others, the classification is preferably
performed with the vibrating sieve apparatus. Examples of the
vibrating sieve apparatus include a dry type vibrating sieve
apparatus [manufactured by KOEISANGYO Co., Ltd., trade name "SATO'S
SYSTEM VIBRO SEPARATOR"].
[0112] The opening of the sieve for use in the classification can
be selected appropriately according to the application of a
thermally conductive member in which a resin composition comprising
the hBN powder to be obtained is used.
[0113] In the case where the hBN powder according to the present
invention is used in a resin sheet, the opening of the sieve can be
selected appropriately according to the film thickness of the resin
sheet and is preferably 50 .mu.m or more and 150 .mu.m or less,
more preferably 60 .mu.m or more and 130 .mu.m or less, still more
preferably 75 .mu.m or 106 .mu.m, further still more preferably 106
.mu.m. In addition, the sieving time can be selected appropriately
according to the opening of the sieve to be used and the amount to
be charged in the apparatus, and in the case where a sieve having
an opening of, for example, 106 .mu.m is used, a powder is
preferably made to pass through the sieve having an opening of 106
.mu.m, the powder obtained through classification under a condition
of a sieving time of 60 minutes.
[0114] The hBN powder according to the present invention is
preferably obtained by the production method further performing at
least one of pulverization or classification after the firing step,
and comprises the hBN powder having a particle size of 45 .mu.m or
more and 106 .mu.m or less.
(Mixing of hBN Powder)
[0115] Further, after the calcined product obtained through the
calcination is pulverized, the method for producing the hBN powder
according to the present invention preferably comprises a step of
classifying the fired product, which is pulverized, using a sieve
having an opening of 106 .mu.m, a sieve having an opening of 45
.mu.m, and a vibrating sieve apparatus into the hBN powder of 45 to
106 .mu.m (hereinafter, also referred to as "hBN powder (.alpha.)"
and the hBN powder passing through the sieve of 45 .mu.m
(hereinafter, also referred to as "hBN powder (.beta.)" and then
mixing the hBN powder (.alpha.) and the hBN powder (.beta.) so that
the ratio of the hBN powder (.alpha.) to the total mass of the hBN
powders (.alpha.) and (.beta.) (hereinafter, also referred to as
granule rate (%)=[.alpha./[(.alpha.)+(.beta.)]]) can be 40% by mass
or more and 95% by mass or less, from the viewpoint of suppressing
the anisotropy and the viewpoint of improvements in the thermal
conductive properties. The mixing method is not particularly
limited, and any of wet mixing and dry mixing may be used, but the
dry mixing is preferable. The dry mixing can be performed using a
general mixer such as a Henschel mixer, a ball mill, a ribbon
blender, or a V-type blender, but the V-type blender is preferable
from the viewpoint of mixing the hBN powder uniformly. The mixing
time is preferably 20 to 90 minutes, more preferably 50 to 70
minutes.
[0116] The granule rate is more preferably 50% by mass or more and
95% by mass or less, still more preferably 60% by mass or more and
90% by mass or less, and further still more preferably 70% by mass
or more and 85% by mass or less from the viewpoint of suppressing
the anisotropy and the viewpoint of improvements in the thermal
conductive properties.
[Classified Hexagonal Boron Nitride Powder]
[0117] In the case of a thin-film resin sheet having a film
thickness of 110 .mu.m or less, the resin sheet obtained by molding
the resin composition comprising the hexagonal boron nitride powder
(hBN powder), the hBN powder is preferably further classified into
the classified hexagonal boron nitride powder (hereinafter, also
simply referred to as "classified hBN powder"). The classification
method is not particularly limited, and the classification can be
performed with the vibrating sieve apparatus or by the air flow
classification, the water sieving, the centrifugal separation, or
the like in the same manner as the hBN powder. Among others, the
classification is preferably performed with the vibrating sieve
apparatus. Examples of the vibrating sieve apparatus include the
dry type vibrating sieve apparatus [manufactured by KOEISANGYO Co.,
Ltd., trade name "SATO'S SYSTEM VIBRO SEPARATOR"].
[0118] The opening of the sieve for use in the classification can
be selected appropriately according to the film thickness of a
thin-film resin sheet obtained by molding a resin composition
comprising the hBN powder to be obtained and is preferably 20 .mu.m
or more and less than 50 .mu.m, more preferably 30 .mu.m or more
and less than 50 .mu.m, and still more preferably 45 .mu.m.
[Resin Composition]
[0119] The resin composition according to the present invention
comprises 10% by volume or more and 90% by volume or less of the
hexagonal boron nitride powder (hBN powder) as a filler. The
content of the hBN powder in the resin composition according to the
present invention is 10% by volume or more and 90% by volume or
less, preferably 20% by volume or more and 80% by volume or less,
more preferably 30% by volume or more and 70% by volume or less,
still more preferably 35% by volume or more and 65% by volume or
less, and further still more preferably 40% by volume or more and
60% by volume or less from the viewpoint of ease of production in
the process of forming a composite with a resin and improvements in
the thermal conductive properties.
[0120] By using the hBN powder, the aggregate can maintain the
granular shape without disintegrating in the process of forming a
composite with a resin in producing the resin composition, and
therefore the filling rate in the resin composition can be
improved, and as a result, the high thermal conductive properties
can be exhibited. Further the hBN powder comprises a strong
aggregate, and therefore the anisotropy due to the disintegration
of the aggregate can be suppressed.
[0121] In the present invention, the content based on volume (% by
volume) of the hBN powder can be determined from the specific
gravity of the hBN powder and the specific gravities of various
resins for use as an organic matrix.
[0122] In addition, in the case where the resin sheet is a
thin-film resin sheet having a film thickness of 110 .mu.m or less
as described below, the classified hBN powder obtained by further
classifying the hBN powder with the vibrating sieve apparatus or
the like is preferably used in the resin composition according to
the present invention. The content of the classified hBN powder (%
by volume) in the resin composition according to the present
invention is the same as the content of the hBN powder.
<Organic Matrix>
[0123] The resin composition according to the present invention
comprises a resin as an organic matrix.
[0124] The resin for use in the present invention preferably
comprises at least one resin selected from the group consisting of
thermosetting resins, thermoplastic resins, various kinds of
rubber, thermoplastic elastomers, oil, and the like.
[0125] Examples of the thermosetting resins include epoxy resins,
silicone resins, phenol resins, urea resins, unsaturated polyester
resins, melamine resins, polyimide resins, polybenzoxazole resins,
and urethane resins.
[0126] Examples of the thermoplastic resins include: polyolefin
resins such as polyethylene, polypropylene, and ethylene-vinyl
acetate copolymers; polyester resins such as polyethylene
terephthalate, polybutylene terephthalate, and liquid crystal
polyesters; and polyvinyl chloride resins, acrylic resins,
polyphenylene sulfide resins, polyphenylene ether resins, polyamide
resins, polyamideimide resins, and polycarbonate resins.
[0127] Examples of the various kinds of rubber include natural
rubber, polyisoprene rubber, styrene-butadiene copolymer rubber,
polybutadiene rubber, ethylene-propylene copolymers,
ethylene-propylene-diene copolymers, butadiene-acrylonitrile
copolymers, isobutylene-isoprene copolymers, chloroprene rubber,
silicone rubber, fluororubber, chloro-sulfonated polyethylenes, and
polyurethane rubber. These kinds of rubber are preferably
crosslinked and used.
[0128] Examples of the thermoplastic elastomers include
olefin-based thermoplastic elastomers, styrene-based thermoplastic
elastomers, vinyl chloride-based thermoplastic elastomers,
urethane-based thermoplastic elastomers, and ester-based
thermoplastic elastomers.
[0129] Examples of the oil component include grease such as
silicone oil.
[0130] The organic matrices may be used singly or in a combination
or two or more.
[0131] The resin for use as the organic matrix can be selected
appropriately according to the application of a thermally
conductive member obtained using the resin composition according to
the present invention and demand characteristics such as the
mechanical strength, heat resistance, durability, softness, and
flexibility of the thermally conductive member.
[0132] Among these, at least one resin selected from the group
consisting of various thermosetting resins, thermoplastic resins,
rubber, and thermoplastic elastomers, and the like which are used
as the organic matrix of the conventional resin sheets, more
preferably thermosetting resins, and still more preferably curable
epoxy resins and curable silicone resins from the viewpoint of
suppressing the anisotropy and the viewpoint of improvements in the
thermal conductive properties.
[0133] The content of the organic matrix in the resin composition
is preferably 10% by volume or more and 90% by volume or less, more
preferably 20% by volume or more and 80% by volume or less, still
more preferably 30% by volume or more and 70% by volume or less,
further still more preferably 35% by volume or more and 65% by
volume or less, and further still more preferably 40% by volume or
more and 60% by volume or less from the viewpoint of the ease of
production in the process of forming a composite with a resin and
the thermal conductive properties.
[0134] In the present invention, the content based on volume (% by
volume) of the organic matrix can be determined from the specific
gravity of the hBN powder and specific gravities of various resins
for use as the organic matrix.
[Curable Epoxy Resin]
[0135] In the resin composition according to the present invention,
as the curable epoxy resin for use as the organic matrix, epoxy
resins which are in a liquid form at normal temperature (namely,
25.degree. C.) and low softening point epoxy resins which are in a
solid form at normal temperature (namely, 25.degree. C.) are
preferable from the viewpoint of dispersibility of the hBN powder
to the organic matrix.
[0136] The curable epoxy resin is not particularly limited as long
as the curable epoxy resin is a compound having two or more epoxy
groups in one molecule, and any of the publicly known compounds
which have been used conventionally as the epoxy resin can be
selected and used appropriately. Examples of such an epoxy resin
include bisphenol A type epoxy resins, bisphenol F type epoxy
resins, glycidyl ethers of a polycarboxylic acid, and epoxy resins
obtained through epoxidation of a cyclohexane derivative. These may
be used singly or in a combination of two or more. Among the epoxy
resins, bisphenol A type epoxy resins, bisphenol F type epoxy
resins, and epoxy resins obtained through epoxidation of a
cyclohexane derivative are suitable from the viewpoint of the heat
resistance, workability, and the like.
(Curing Agent for Epoxy Resin)
[0137] A curing agent for epoxy resins is usually used for curing
the curable epoxy resin. The curing agent for epoxy resins is not
particularly limited, any of the curing agents which have been used
conventionally as the curing agent for epoxy resins can be selected
and used appropriately, and examples thereof include amine-based,
phenol-based, acid anhydride-based and imidazole-based curing
agents. Examples of the amine-based curing agents preferably
include dicyandiamide and aromatic diamines such as
m-phenylenediamine, 4,4'-diaminodiphenylmethane,
4,4'-diaminodiphenylsulfone, and m-xylylenediamine. Examples of the
phenol-based curing agents preferably include phenol novolac
resins, cresol novolac resins, bisphenol A type novolac resins, and
triazine-modified phenol novolac resins. In addition, examples of
the acid anhydride-based curing agents include alicyclic acid
anhydrides such as methylhexahydrophthalic anhydride, aromatic acid
anhydrides such as phthalic anhydride, aliphatic acid anhydrides
such as aliphatic dibasic acid anhydrides, and halogen-based acid
anhydrides such as chlorendic anhydride. Examples of the
imidazole-based curing agents include 2-methylimidazole,
2-undecylimidazole, 2-heptadecylimidazole,
2-ethyl-4-methylimidazole, 2-phenylimidazole,
2-phenyl-4-methylimidazole, and
1-cyanoethyl-2-ethyl-4-methylimidazole.
[0138] These curing agents may be used singly or in a combination
of two or more. The amount of the curing agent for epoxy resins to
be used is usually selected in a range of an equivalent ratio of
about 0.5 to about 1.5, preferably in a range of an equivalent
ratio of 0.7 to 1.3 in terms of the equivalent ratio of the curing
agent to the curable epoxy resin from the viewpoint of curability,
a balance among physical properties of a cured resin, and the
like.
(Curing Accelerator for Epoxy Resins)
[0139] In the resin composition according to the present invention,
a curing accelerator for epoxy resins can be used as necessary
together with the curing agent for epoxy resins.
[0140] The curing accelerator for epoxy resins is not particularly
limited, any of the curing accelerators which have been used
conventionally as the curing accelerator for epoxy resins can be
selected and used appropriately. Examples include imidazole
compounds such as 2-ethyl-4-methylimidazole,
1-benzyl-2-methylimidazole, 2-methylimidazole, 2-ethylimidazole,
2-isopropylimidazole, 2-phenylimidazole, and
2-phenyl-4-methylimidazole, 2,4,6-tris(dimethylaminomethyl)phenol,
boron trifluoride-amine complexes, and triphenylphosphine. These
curing accelerators may be used singly or in a combination of two
or more. The amount of the curing accelerator for epoxy resins to
be used is usually selected in a range of about 0.1 to about 10
parts by mass, preferably in a range of 0.4 to 5 parts by mass
based on 100 parts by mass of the curable epoxy resin from the
viewpoint of curing acceleration properties, the balance among
physical properties of the cured resin, and the like.
[Curable Silicone Resin]
[0141] As the curable silicone resin, a mixture of an addition
reaction type silicone resin and a silicone-based crosslinking
agent can be used. Examples of the addition reaction type silicone
resin include at least one selected from the group consisting of
polyorganosiloxanes comprising an alkenyl group as a functional
group in the molecule. Preferred examples of the
polyorganosiloxanes comprising an alkenyl group as a functional
group in the molecule include a polydimethylsiloxane comprising a
vinyl group as a functional group, a polydimethylsiloxane
comprising a hexenyl group as a functional group, and a mixture
thereof.
[0142] Examples of the silicone-based crosslinking agent include
polyorganosiloxanes comprising at least 2 silicon atom-bonded
hydrogen atoms in one molecule, specifically,
dimethylsiloxane-methylhydrogensiloxane copolymers end-capped with
a dimethylhydrogensiloxy group,
dimethylsiloxane-methylhydrogensiloxane copolymers end-capped with
a trimethylsiloxy group, poly(methylhydrogensiloxane) end-capped
with a trimethylsiloxane group, and poly(hydrogen
silsesquioxane).
[0143] In addition, as a curing catalyst, a platinum-based compound
is usually used. Examples of the platinum-based compound include
particulate platinum, particulate platinum adsorbed on a carbon
powder carrier, chloroplatinic acid, alcohol-modified
chloroplatinic acid, olefin complexes of chloroplatinic acid,
palladium, and rhodium catalysts.
[0144] The resin composition according to the present invention may
further comprise another component in a range where the effects of
the present invention are obtained. Examples of such a component
include a particle of a nitride such as aluminum nitride, silicon
nitride, and fibrous boron nitride, electrically insulating metal
oxides such as alumina, fibrous alumina, zinc oxide, magnesium
oxide, beryllium oxide, and titanium oxide, electrically insulating
carbon components such as diamond and fullerene, a plasticizing
agent, an adhesive, a reinforcing agent, a coloring agent, a heat
resistance improver, a viscosity modifier, a dispersion stabilizer,
and a solvent.
[0145] Moreover, in the resin composition according to the present
invention, an inorganic filler such as aluminum hydroxide or
magnesium hydroxide, a surface treating agent such as a silane
coupling agent which improves the adhesion strength at an interface
between the inorganic filler and the resin, a reducing agent, or
the like may be added in addition to the materials each listed as
an example of the nitride particle and the electrically insulating
metal oxide as long as the effects of the present invention are not
impaired.
[0146] The resin composition according to the present invention can
be produced, for example, in the manner as described below.
[0147] The organic matrix is first prepared by mixing the resin,
and the curing agent and the solvent as necessary.
[0148] Subsequently, the hBN powder is added to the organic matrix
so that the hBN powder can be contained in a proportion of 10% by
volume or more and 90% by volume or less in the total amount of the
hBN powder and the organic matrix. The weight of the hBN powder and
of the resin are each set according to the specific gravity of the
hBN powder and the specific gravity of the resin to be used as the
organic matrix so that a desired % by volume of the hBN powder and
of the resin can be contained, and the hBN powder and the resin are
weighed and then mixed to prepare the resin composition.
[0149] In the case where the curable epoxy resin is used as a main
component of the organic matrix in the resin composition according
to the present invention, a mixture of the curable epoxy resin, the
curing agent for epoxy resins, and the curing accelerator for epoxy
resins which is used as necessary forms the organic matrix. In
addition, in the case where the curable silicone resin is used as a
main component of the organic matrix, a mixture of the addition
reaction type silicone resin, the silicone-based crosslinking
agent, and the curing catalyst forms the organic matrix.
[0150] The resin composition which is obtained in this way can be
used for a thermally conductive member such as a thermally
conductive sheet, thermally conductive gel, thermally conductive
grease, a thermally conductive adhesive, or a phase change sheet.
As a result, the heat from a heat generating electronic component
such as an MPU, a power transistor, or a transformer can be
transferred efficiently to a heat dissipation component such as a
heat dissipation fin or a heat dissipation fan.
[0151] Among the thermally conductive members, the resin
composition is preferably used as a thermally conductive sheet and
for a resin sheet. By using the resin composition for a resin
sheet, the effects of the resin composition can be particularly
exhibited from the viewpoint of suppressing the anisotropy and the
viewpoint of improvements in the thermal conductive properties.
[Resin Sheet]
[0152] The resin sheet according to the present invention comprises
the resin composition or a cured product thereof and is obtained by
molding the resin composition into a sheet. In the case where the
resin composition is curable, the resin sheet according to the
present invention is obtained by molding the resin composition into
a sheet and then curing the molded resin composition.
[0153] The resin sheet according to the present invention can be
produced by applying the resin composition on a base material, such
as a releasable film including a resin film with a release layer,
or the like, with a usual coating machine or the like, and, in the
case where the resin composition comprises a solvent, then drying
the solvent with a far infrared ray radiation heater, or by hot air
blowing or the like to form a sheet.
[0154] As the release layer, a melamine resin or the like is used.
In addition, as the resin film, a polyester resin or the like such
as polyethylene terephthalate is used.
[0155] In the case where the organic matrix in the resin
composition is not a curable organic matrix such as the curable
epoxy resin or the curable silicone resin, the resin sheet per se
which is formed into a sheet is the resin sheet according to the
present invention.
[0156] Further, in the case where the organic matrix is a curable
matrix, the resin sheet which is obtained above and formed on the
base material is pressurized as necessary through the base material
from a side of a surface of the base material, the surface not
coated with the resin composition, and is then further subjected to
a heat treatment to be cured to obtain the resin sheet according to
the present invention. The pressurization condition is preferably
15 MPa or more and 20 MPa or less, more preferably 17 MPa or more
and 19 MPa or less. In addition, the heat condition is preferably
80.degree. C. or more and 200.degree. C. or less, more preferably
100.degree. C. or more and 150.degree. C. or less. It is to be
noted that the base material for the releasable film and the like
is usually peeled or removed finally.
[0157] The film thickness of the resin sheet according to the
present invention which is obtained in this way is preferably 50
.mu.m or more and 10 mm or less, more preferably 50 .mu.m or more
and 1.0 mm or less, still more preferably 50 .mu.m or more and 500
.mu.m or less, further still more preferably 60 .mu.m or more and
400 .mu.m or less, and further still more preferably 70 .mu.m or
more and 300 .mu.m or less from the viewpoint of moldability.
Moreover, the film thickness of the resin sheet according to the
present invention is preferably in a range of 50 .mu.m or more and
150 .mu.m or less, more preferably 60 .mu.m or more and 130 .mu.m
or less, and still more preferably 70 .mu.m or more and 110 .mu.m
or less from the viewpoint of reducing the weight and thickness of
electronic components and the like for which the resin sheet is
used.
[0158] In the case where the resin sheet is a thin-film resin sheet
having a film thickness of 110 .mu.m or less, the resin composition
comprising the classified hBN powder obtained by further
classifying the hBN powder with the vibrating sieve apparatus or
the like is preferably molded.
[0159] The resin sheet according to the present invention has a
thermal conductivity in the thickness direction of preferably 5.0
W/mK or more, more preferably 10 W/mK or more, still more
preferably 15 W/mK or more, further still more preferably 18 W/mK
or more, further still more preferably 20 W/mK, further still more
preferably 22 W/mK or more, further still more preferably 24 W/mK
or more, and further still more preferably 25 W/mK or more.
[0160] Moreover, the resin sheet according to the present invention
preferably has a degree of orientation of the primary particles of
hBN of 25 or less, more preferably 23 or less, still more
preferably 20 or less, further still more preferably 18 or less,
further still more preferably 17 or less, further still more
preferably 16 or less, and further still more preferably 15 or
less.
[0161] The resin sheet according to the present invention has a
specific gravity rate of preferably 90% or more and 100% or less,
more preferably 95% or more and 100% or less, and still more
preferably 98% or more and 100% or less, and further still more
preferably 100% from the viewpoint of the electric insulation.
[0162] The resin sheet according to the present invention may be
used by laminating or embedding a member in a sheet form, a fiber
form, or a net-like appearance on one surface or both surfaces
thereof, or in the sheet, for improving workability or
reinforcement.
[0163] The resin sheet thus obtained can be made to be a product
form for use as a resin sheet in a state where the obtained resin
sheet is peeled from the releasable film or in a state where the
releasable film is used as a protective film.
[0164] Moreover, the resin sheet according to the present invention
may have a configuration in which an adhesive layer is further
provided on the upper surface or the lower surface of the resin
sheet, thereby enhancing convenience during the use of a
product.
[0165] The resin sheet according to the present invention is used,
for example, as a thermally conductive sheet with which the heat
from a heat generating electronic component such as an MPU, a power
transistor, or a transformer is transferred to a heat dissipation
component such as a heat dissipation fin or a heat dissipation fan,
and is used by being interposed between the heat generating
electronic component and the heat dissipation component. Thereby,
the heat transfer between the heat generating electronic component
and the heat dissipation component becomes good and malfunction of
the heat generating electronic component can be reduced
remarkably.
EXAMPLES
[0166] Hereinafter, the present invention will be described further
specifically giving Examples and Comparative Examples, but the
present invention in not limited by these examples.
Example 1
(1) Preparation of Mixed Powder
[0167] As the BN fine powder (A), 65 parts by mass of a BN fine
powder (A-1) having the following properties and 35 parts of boron
oxide (B.sub.2O.sub.3, boric anhydride) manufactured by KANTO
CHEMICAL CO., INC. as the boron compound (B) were mixed using a
mixer to obtain a mixed powder (X1) having a content of the BN fine
powder (A) of 65% by mass and a content of the boron compound (B)
of 35% by mass.
[0168] BN fine powder (A-1): ratio [L.sub.2/d.sub.2] 6.0, D.sub.50
0.67 .mu.m, BET specific surface area 9.9 m.sup.2/g, crystallite
size 262 .ANG.
(2) Preparation of hBN Powder
[0169] As the carbon source (C), 3.3 parts by mass of boron carbide
(B.sub.4C) manufactured by RIKEN CORUNDUM CO., LTD. and 10 parts by
mass of an aqueous PVA solution (concentration of 2.5% by mass)
were added based on 100 parts by mass of the mixed power (X1) to
obtain a mixture having a content of the carbon source (C) in terms
of carbon of 3.3 parts by mass based on 100 parts by mass of the
mixed powder. The mixture was stirred and mixed with a mixer,
thereafter put into a metal mold, and then pressurized to obtain a
tablet-like molded body having a density of 1.4 g/cm.sup.3. The
molded body was dried in a dryer at 300.degree. C. for 6 hours to
obtain a dried product. The dried product was fired in a
high-frequency furnace at 1750.degree. C. to 2200.degree. C. under
a nitrogen gas atmosphere for 6 hours in total to obtain an hBN
fired product.
[0170] The obtained hBN fired product was pulverized using a jaw
crusher and a pin mill and was then classified using the dry type
vibrating sieve apparatus [manufactured by KOEISANGYO Co., Ltd.,
trade name "SATO'S SYSTEM VIBRO SEPARATOR"] with a sieve having an
opening of 106 .mu.m and a sieve having an opening of 45 .mu.m
under a condition of a sieving time of 60 minutes into the hBN
powder (.alpha.) of 45 to 106 .mu.m and the hBN powder (.beta.)
passing through the sieve of 45 .mu.m, and the powder of exceeding
106 .mu.m was removed.
[0171] The hBN powder (.alpha.) and the hBN powder (.beta.) were
mixed so that the granule rate was 80% by mass to obtain an hBN
powder according to Example 1.
[0172] It is to be noted that the granule rate is expressed by the
following expression, namely, by the ratio of the hBN powder
(.alpha.) to the total mass of the hBN powder (.alpha.) of 45 to
106 .mu.m and the hBN powder (.beta.) passing through the sieve of
45 .mu.m.
Granule rate (%)=[(.alpha.)/[(.alpha.)+(.beta.)]]
[0173] When the obtained hBN powder was observed with an SEM, it
was ascertained that the hBN powder comprises an hBN aggregate in
which each primary particle faces in a random direction as shown in
FIG. 2 and FIG. 3. It is to be noted that FIG. 1 is a schematic
diagram of the hBN aggregate existing in FIG. 2 and in FIG. 3.
(3) Preparation of Resin Composition
[0174] Firstly, 100 parts by mass of a curable liquid epoxy resin
[manufactured by Japan Epoxy Resin, trade name "jER 828", bisphenol
A type, epoxy equivalence of 184 to 194 g/eq] and 5 parts by mass
of 1-cyanoethyl-2-ethyl-4-methylimidazole [manufactured by SHIKOKU
CHEMICALS CORPORATION, trade name "2E4MZ-CN"] as a curing agent
were mixed to prepare an organic matrix.
[0175] Subsequently, the hBN powder obtained above was added
thereto based on 100 parts by mass of the organic matrix described
above so that the hBN powder content in the total amount of the hBN
powder and the organic matrix was 60% by volume, and the resultant
mixture was stirred and mixed using MAZERUSTAR.RTM. manufactured by
KURABO INDUSTRIES LTD. to prepare a resin composition.
[0176] It is to be noted that the content based on volume (% by
volume) of the hBN powder was determined from the specific gravity
of the hBN powder (2.27) and the specific gravity of the curable
liquid epoxy resin (1.17) used as the organic matrix.
(4) Preparation of Resin Sheet
[0177] Molding was performed using the resin composition obtained
above and a metallic mold on a releasable film cut to 10.5 cm wide
and 13 cm length so that the cured film thickness was 500 .mu.m or
less. Thereafter, the molded resin composition was interposed
between releasable films, and then crimping was performed on the
molded resin composition through the releasable films with a
metallic mold under conditions of 120.degree. C. and 18 MPa for 10
minutes to cure the resin composition, thereby preparing a resin
sheet.
Example 2
[0178] An hBN powder, a resin composition, and a resin sheet were
each prepared in the same manner as in Example 1 except that 5.0
parts by mass of artificial graphite fine powder "UF-G30"
manufactured by Showa Denko K.K. and 2.0 parts by mass of the boron
carbide based on 100 parts by mass of the mixed powder (X1) were
used as the carbon source (C) in Example 1 (2).
Example 3
[0179] An hBN powder, a resin composition, and a resin sheet were
each prepared in the same manner as in Example 1 except that 2.2
parts by mass of the boron carbide based on 100 parts by mass of
the mixed powder (X1) was used as the carbon source (C) in Example
1 (2).
Example 4
[0180] An hBN powder, a resin composition, and a resin sheet were
each prepared in the same manner as in Example 1 except that 10
parts by mass of the artificial graphite fine powder based on 100
parts by mass of the mixed powder (X1) was used as the carbon
source (C) in Example 1 (2).
Example 5
(1) Preparation of Mixed Powder
[0181] As the BN fine powder (A), 80 parts by mass of a BN fine
powder (A-1) and 20 parts by mass of the boron oxide
(B.sub.2O.sub.3, boric anhydride) as the boron compound (B) were
mixed using a mixer to obtain a mixed powder (X2) having a content
of the BN fine powder (A) of 80% by mass and a content of the boron
compound (B) of 20% by mass.
(2) Preparation of hBN Powder, Resin Composition, and Resin
Sheet
[0182] An hBN powder, a resin composition, and a resin sheet were
each prepared in the same manner as in Example 1 (2) to (4) except
that the mixed powder (X2) was used in place of the mixed powder
(X1), and 1.6 parts by mass of the boron carbide based on 100 parts
by mass of the mixed powder (X2) was used as the carbon source (C)
in Example 1 (2).
Example 6
(1) Preparation of Mixed Powder
[0183] As the BN fine powder (A), 65 parts by mass of a BN fine
powder (A-2) having the following properties and 35 parts by mass
of the boron oxide (B.sub.2O.sub.3, boric anhydride) as the boron
compound (B) were mixed using a mixer to obtain a mixed powder (X4)
having a content of the BN fine powder (A) of 65% by mass and a
content of the boron oxide (B) of 35% by mass.
[0184] BN fine powder (A-2): ratio [L.sub.2/d.sub.2] 8.0, D.sub.50
4.50 .mu.m, BET specific surface area 11.7 m.sup.2/g, crystallite
size 203 .ANG.
(2) Preparation of hBN Powder, Resin Composition, and Resin
Sheet
[0185] An hBN powder, a resin composition, and a resin sheet were
each prepared in the same manner as in Example 1 (2) to (4) except
that the mixed powder (X4) was used in place of the mixed powder
(X1), and 3.3 parts by mass of the boron carbide based on 100 parts
by mass of the mixed powder (X4) was used as the carbon source (C)
in Example 1 (2).
Example 7
(1) Preparation of Mixed Powder
[0186] As the BN fine powder (A), 65 parts by mass of a BN fine
powder (A-3) having the following properties and 35 parts by mass
of the boron oxide (B.sub.2O.sub.3, boric anhydride) as the boron
compound (B) were mixed using a mixer to obtain a mixed powder (X5)
having a content of the BN fine powder (A) of 65% by mass and a
content of the boron oxide (B) of 35% by mass.
[0187] BN fine powder (A-3): ratio [L.sub.2/d.sub.2] 7.5, D.sub.50
0.40 .mu.m, BET specific surface area 26.0 m.sup.2/g, crystallite
size 161 .ANG.
(2) Preparation of hBN Powder, Resin Composition, and Resin
Sheet
[0188] An hBN powder, a resin composition, and a resin sheet were
each prepared in the same manner as in Example 1 (2) to (4) except
that the mixed powder (X5) was used in place of the mixed powder
(X1), and 3.3 parts by mass of the boron carbide based on 100 parts
by mass of the mixed powder (X5) was used as the carbon source (C)
in Example 1 (2).
Comparative Example 1
[0189] A resin composition and a resin sheet were each prepared in
the same manner as in Example 1 except that an hBN powder was
prepared in the manner as described below in place of Example 1 (1)
and (2).
(1-1) Preparation of Crude hBN Powder
[0190] A mixture obtained by adding 4 g of boric acid, 2 g of
melamine, and 1 g of water was stirred and mixed, and the resultant
mixture was put into a metal mold and then pressurized to obtain a
molded body having a density of 0.7 g/cm.sup.3. A dried product
obtained by drying the molded body in a dryer at 300.degree. C. for
100 minutes was calcined at 1100.degree. C. under an NH.sub.3 gas
atmosphere for 120 minutes. The calcined product thus obtained
(crude hBN) was pulverized to obtain a crude hBN powder (content of
boron oxide of 35% by mass).
(2-1) Preparation of hBN Powder
[0191] As the carbon source, 10 parts by mass of the graphite fine
powder "UF-G30", 0.4 parts by mass of calcium carbonate as the Ca
compound, and 10 parts by mass of an aqueous PVA solution
(concentration of 2.5% by mass) were added based on 100 parts by
mass of the crude hBN powder to obtain a mixture having a content
of the carbon source in terms of carbon of 10 parts by mass based
on 100 parts by mass of the crude hBN powder. The mixture was
stirred and mixed with a mixer, thereafter put into a metal mold,
and then pressurized to obtain a molded body having a density of
1.2 g/cm.sup.3. The molded body was dried in a dryer at 300.degree.
C. for 6 hours to obtain a dried product. The dried product was
fired in a high-frequency furnace at 1750.degree. C. to
2200.degree. C. under a nitrogen gas atmosphere for 6 hours in
total to obtain an hBN fired product.
[0192] The obtained hBN fired product was pulverized using a jaw
crusher and a pin mill and was then classified using the dry type
vibrating sieve apparatus [manufactured by KOEISANGYO Co., Ltd.,
trade name "SATO'S SYSTEM VIBRO SEPARATOR"] with a sieve having an
opening of 106 .mu.m and a sieve having an opening of 45 .mu.m
under a condition of a sieving time of 60 minutes into the hBN
powder (.alpha.) of 45 to 106 .mu.m and the hBN powder (.beta.)
passing through the sieve of 45 .mu.m.
[0193] The hBN powder (.alpha.) and the hBN powder (.beta.) were
mixed so that the granule rate was 80% by mass to obtain an hBN
powder according to Comparative Example 1.
[0194] It is to be noted that the granule rate is expressed by the
following expression, namely, by the ratio of the hBN powder
(.alpha.) to the total mass of the hBN powder (.alpha.) of 45 to
106 .mu.m and the hBN powder (.beta.) passing through the sieve of
45 .mu.m.
Granule rate (%)=[(.alpha.)/[(.alpha.)+(.beta.)]]
Comparative Example 2
[0195] A resin composition and a resin sheet were each prepared in
the same manner as in Example 1 except that an hBN powder "UHP-EX"
manufactured by Showa Denko K.K. was used in place of the hBN
powder obtained in Example 1 (1) and (2).
Comparative Example 3
[0196] An hBN powder, a resin composition, and a resin sheet were
each prepared in the same manner as in Example 1 except that a
mixed powder (XC-1) was made using the BN fine powder (AC-1)
described below in place of the BN fine powder (A-1) in Example 1
(1).
[0197] BN fine powder (AC-1): ratio [L.sub.2/d.sub.2] 22.0,
D.sub.50 9.90 .mu.m, BET specific surface area 17.6 m.sup.2/g,
crystallite size 197 .ANG.,
Comparative Example 4
(1) Preparation of Mixed Powder
[0198] As the BN fine powder (A), 45 parts by mass of the BN fine
powder (A-1), 55 parts by mass of the boron oxide (B.sub.2O.sub.3,
boric anhydride) as the boron compound (B) were mixed using a mixer
to obtain a mixed powder (X3) having a content of the BN fine
powder (A) of 45% by mass and a content of the boron compound (B)
of 55% by mass.
(2) Preparation of hBN Powder, Resin Composition, and Resin
Sheet
[0199] An hBN powder, a resin composition, and a resin sheet were
each prepared in the same manner as in Example 1 (2) to (4) except
that the mixed powder (X3) was used in place of the mixed powder
(X1), and 2.2 parts by mass of the boron carbide based on 100 parts
by mass of the mixed powder (X3) was used as the carbon source (C)
in Example 1 (2).
[Evaluation]
[0200] The following evaluations were conducted for the BN fine
powders, the hBN powders, the resin compositions, and the resin
sheets. The evaluation results are shown in Table 2.
(Diameter of Primary Particles of hBN Powder)
[0201] An SEM photograph was taken for the hBN powder obtained in
each of Examples and Comparative Examples, and the longer diameters
were measured for 100 hBN primary particles arbitrarily selected
from the primary particles in the SEM photograph, and the number
average value of the longer diameters was determined as the
diameter of the primary particles of the hBN powder.
(Ratio [L.sub.2/d.sub.2] of BN Fine Powder and Ratio
[L.sub.1/d.sub.1] of hBN Powder)
[0202] An SEM photograph was taken for the BN fine powder used in
each of Examples and Comparative Examples and for the hBN powder
obtained in each of Examples and Comparative Examples, and the
longer diameters and the shorter diameters were measured for 100
primary particles arbitrarily selected from the primary particles
in the SEM photograph. The number average value of the longer
diameters was determined as the average longer diameter (L.sub.2)
or average longer diameter (L.sub.1) of the primary particles, and
the number average value of the thicknesses was determined as the
average thickness (d.sub.2) or average thickness (d.sub.1) of the
primary particles to calculate the ratio of the average longer
diameter to the average thickness of the primary particles,
[L.sub.2/d.sub.2] and [L1/d.sub.1].
(BET Specific Surface Area of BN Fine Powder and of hBN Powder)
[0203] The BET specific surface area was measured for the BN fine
powders used in Examples and Comparative Examples and the hBN
powders obtained in Examples and Comparative Examples by the BET
one-point method utilizing the fluid process using a full-automatic
BET specific surface area measuring apparatus [manufactured by
Yuasa Ionics Inc., model name "Multisorb 16"].
(Peak Decrease Rate)
[0204] In the present invention, the measurement of the peak
decrease rate was conducted using a particle size distribution
analyzer [manufactured by NIKKISO CO., LTD., model name "Microtrac
MT3300EX II"] of the laser diffraction scattering method.
[0205] A dispersion liquid was prepared in such a way that the hBN
powder of each of Examples and Comparative Examples was classified
with the dry type vibrating sieve apparatus (sieving time of 60
minutes) using a sieve having an opening of 106 .mu.m and a sieve
having an opening of 45 .mu.m with the two sieves piled up, and
0.06 g of the classified hBN powder having a particle size of 45
.mu.m or more and 106 .mu.m or less was then dispersed in 50 g of
water. The dispersion liquid was placed in a 50-ml container and
was then subjected to an ultrasonic treatment for 1 minute under
conditions of an output of 150 W and an oscillating frequency of
19.5 kHz, and thereafter the measurement of the particle size
distribution was conducted while stirring the dispersion liquid
using a magnetic stirrer under a condition of a number of
revolutions of 400 rpm. The maximum peak that appeared between a
particle size of 45 .mu.m or more and a particle size of 150 .mu.m
or less after the ultrasonic treatment and the maximum peak that
appeared between a particle size of 45 .mu.m or more and a particle
size of 150 .mu.m or less before the ultrasonic were compared.
[0206] FIG. 6 is a graph showing the particle size distribution
curves of Example 1. In this figure, the peak decrease rate
[=[(maximum peak height before treatment (a))-(maximum peak height
after treatment (b))]/(maximum peak height before treatment (a))]
was calculated. The broken line in the figure shows a graph showing
the particle size distribution curve of the hBN powder before the
ultrasonic treatment, and the solid line shows a graph showing the
particle size distribution curve of the hBN powder after the
ultrasonic treatment. It can be said that the lower the peak
decrease rate is, the higher the disintegration strength is. It is
to be noted that the ultrasonic treatment in the present invention
was performed using an ultrasonic treatment apparatus [manufactured
by NIHONSEIKI KAISHA LTD., model name "Ultrasonic Homogenizer
US-150V"].
(Crystallite Size of BN Fine Powder and of hBN Powder)
[0207] The crystallite size of the BN fine powder used in each of
Examples and Comparative Examples and of the hBN powder obtained in
each of Examples and Comparative Examples were calculated through
X-ray diffraction measurement. As an X-ray diffraction measuring
apparatus, a model name "X'Pert PRO" manufactured by PANalytical
B.V. was used, and Cu--K.alpha.-1 line was used by use of a Cu
target.
(Bulk Density of hBN Powder)
[0208] The hBN powder obtained in each of Examples and Comparative
Examples in an amount of 100 g was put into a 300-ml measuring
flask to tap the cylinder 50 times in a row from a height of 1 cm,
and the tap density calculated by reading a volume value when the
value did not vary for 3 sets in a row was measured as the bulk
density.
(Density of Molded Body of hBN Powder)
[0209] The mass and the volume of each molded body were measured,
and the density of the molded body was determined from these
values.
(50% Volume Cumulative Particle size (D.sub.50) of BN Fine Powder
and hBN Powder)
[0210] The 50% volume cumulative particle size (D.sub.50) was
measured using the particle size distribution analyzer
[manufactured by NIKKISO CO., LTD., model name "Microtrac MT3300EX
II"].
[0211] The measurement of the particle size distribution was
conducted using a dispersion liquid prepared by subjecting 0.06 g
of the BN fine powder used in each of Examples and Comparative
Examples or the hBN powder obtained in each of Examples and
Comparative Examples to an ultrasonic treatment in 50 g of pure
water for 3 minutes.
(Content of hBN Powder not Passing Through Sieve Having Opening of
45 .mu.m)
[0212] A sieve having an opening of 45 .mu.m, the sieve having a
diameter of 20 cm, a height of 4.5 cm was prepared, and 10 g of the
hBN powder obtained in each of Examples and Comparative Examples
was put on the sieve to be set in a reduced pressure suction type
sieving machine [manufactured by Alpine Ag., model name "Air Jet
Sieve A 200 LS"]. Sieving was performed by sucking the powder from
under the sieve with a differential pressure of 1 kPa for 180
seconds as the sieving time. The weight of the powder passing
through the sieve and of the powder left on the sieve were measured
to calculate the content of the hBN powder not passing through the
sieve having an opening of 45 .mu.m (content of powder not passing
through sieve having an opening of 45 .mu.m (% by mass)).
(Boron Oxide (B.sub.2O.sub.3) Content and Calcium Oxide (CaO)
Content in hBN Powder)
[0213] The hBN powder obtained in each of Examples and Comparative
Examples was subjected to an acid treatment with 0.1 N a diluted
sulfuric acid solution. Through this acid treatment, boron oxide
(hereinafter, also simply referred to as "B.sub.2O.sub.3") in the
hBN powder dissolves in the acid solution.
[0214] Subsequently, the amount of an B element existing in the
acid solution after the acid treatment was measured with an
apparatus for ICP analysis [manufactured by SII Nano Technology
Inc., model name "SPS 3500"]. The content of B.sub.2O.sub.3 which
had dissolved through the acid treatment was calculated from the
amount of the B element existing in the acid solution after the
acid treatment.
[0215] A Ca element existing in the acid solution after the acid
treatment was measured with the apparatus for ICP analysis, and the
content of CaO was calculated from the amount of the Ca
element.
(Carbon Content in hBN Powder)
[0216] The content of carbon in the hBN powder obtained in each of
Examples and Comparative Examples (carbon content) was measured
using a carbon analyzer [manufactured by LECO Japan Corporation,
model name "C5230"].
(Purity of hBN Powder)
[0217] The total amount of the B.sub.2O.sub.3 content, the CaO
content, and the carbon content in the hBN powder measured as
described above were regarded as the amount of impurities to
determine the purity of the hBN powder.
(Thermal Conductivity of Resin Sheet)
[0218] The thermal diffusivity was measured for the resin sheets
obtained in Examples and Comparative Examples with a model name
"LFA447 NanoFlash" manufactured by Erich NETZSC GmbH & Co.
Holding KG. A value obtained by multiplying the thermal diffusivity
value by theoretical values of the specific heat and the density of
each resin sheet was determined as the thermal conductivity in the
thickness direction of the resin sheet.
[0219] It is to be noted that the theoretical value of the density
of the resin sheet of each of Examples and Comparative Examples was
calculated assuming the theoretical density of boron nitride to be
2.27 g/cm.sup.3 and the theoretical density of the resin component
to be 1.17 g/cm.sup.3.
(Specific Gravity Rate of Resin Sheet)
[0220] The specific gravity rate of the resin sheet obtained in
each of Examples and Comparative Examples was calculated by
dividing the specific gravity of the resin sheet of each of
Examples or Comparative Examples, which was measured using an
electronic balance (model name "CP224S") and specific
gravity/density determination kit (model name
"YDK01/YDK01-OD/YDK01LP") each manufactured by Sartorius
Mechatronics Japan K.K. by an Archimedes method, by the theoretical
specific gravity of the resin sheet of each of Examples or
Comparative Examples, and then multiplying the result by 100,
[(specific gravity measured for resin sheet of each of Examples or
Comparative Examples/theoretical specific gravity of resin sheet of
each of Examples or Comparative Examples).times.100].
[0221] It is to be noted that in the calculation of the theoretical
specific gravity of the resin sheet of each of Examples or
Comparative Examples, the calculation was conducted assuming the
theoretical density of boron nitride to be 2.27 g/cm.sup.3 and the
theoretical density of the resin component to be 1.17
g/cm.sup.3.
(Degree of Orientation)
[0222] The degree of orientation of the resin sheet obtained in
each of Examples and Comparative Examples were calculated by X-ray
diffraction measurement. As an X-ray diffraction measuring
apparatus, a model name "X'Pert PRO" manufactured by PANalytical
B.V. was used, and Cu--K.alpha.-1 line was used by use of a Cu
target.
[0223] The degree of orientation was calculated from the intensity
of a peak corresponding to a 002 plane, the peak appearing at
around a diffraction angle of 2.theta.=26.9 degrees [I(0002)] and
the intensity of a peak corresponding to a 100 plane, the peak
appearing at around a diffraction angle 2.theta.=41.6 degrees
[I(100)] by the following expression wherein the a-axis direction
of the primary particles of the hBN powder is assumed to be the 002
plane and the c-axis direction of the primary particles of the hBN
powder is assumed to the 100 plane.
Degree of orientation=[I(002)/I(100)]
[0224] The conditions for preparing the hBN powders of Examples and
Comparative Examples described above are shown in Table 1-1 and
Table 1-2, and the evaluation results are shown in Table 2.
TABLE-US-00001 TABLE 1-1 BET specific BN fine surface Crystallite
powder (A) Ratio [L.sub.2/d.sub.2] D.sub.50 area size Type -- .mu.m
m.sup.2/g .ANG. A-1 6.0 0.67 9.9 262 A-2 8.0 4.50 11.7 203 A-3 7.5
0.40 26.0 161 AC-1 22.0 9.90 17.6 197
TABLE-US-00002 TABLE 1-2 Mixed powder Carbon source (C) BN fine
Graphite Boron powder (A) fine carbide Total Content Boron compound
(B) powder Parts amount (% by Content Parts by by Parts by Type
Type mass) (*1) (% by mass) (*1) mass (*2) mass (*2) mass (*2)
Example 1 X1 A-1 65 35 0.0 3.3 3.3 Example 2 X1 A-1 65 35 5.0 2.0
7.0 Example 3 X1 A-1 65 35 0.0 2.2 2.2 Example 4 X1 A-1 65 35 10.0
0.0 10.0 Example 5 X2 A-1 80 20 0.0 1.6 1.6 Example 6 X4 A-2 65 35
0.0 3.3 3.3 Example 7 X5 A-3 65 35 0.0 3.3 3.3 Comparative Example
1 -- -- -- -- -- -- -- Comparative Example 2 -- -- -- -- -- -- --
Comparative Example 3 XC-1 AC-1 65 35 0.0 3.3 3.3 Comparative
Example 4 X3 A-1 45 55 0.0 2.2 2.2 (*1) Content in mixed powder
(*2) Amount used in terms of carbon based on 100 parts by mass of
mixed powder
TABLE-US-00003 TABLE 2 Example Comparative Example 1 2 3 4 5 6 7 1
2 3 4 hBN Primary particle size .mu.m 3 6 4 3 5 5 3 10 10 4 3
powder Ratio [L.sub.1/d.sub.1] -- 12 14 8 12 15 15 13 16 14 26 8
BET specific m.sup.2/g 4.5 4.8 4.4 4.7 4.3 3.7 5.3 3.5 3.1 4.2 3.6
surface area Peak decrease rate % 25 21 34 27 30 29 25 57 59 24 45
Crystallite size .ANG. 390 439 387 373 410 344 313 310 456 350 385
Bulk density g/cm.sup.3 0.80 0.83 0.75 0.80 0.81 0.71 0.77 0.60
0.70 0.75 0.53 D.sub.50 .mu.m 42 39 32 41 24 24 30 13 17 40 18
Content of powder % by 82 82 83 81 81 81 82 83 32 82 81 not passing
through mass sieve of 45 .mu.m B.sub.2O.sub.3 content % by 0.05
0.05 0.04 0.04 0.03 0.03 0.04 0.05 0.30 0.07 0.04 mass CaO content
% by 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.60 0.01 0.01 mass
Carbon content % by 0.01 0.02 0.01 0.01 0.02 0.02 0.01 0.01 0.02
0.01 0.01 mass Purity % by 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9
99.1 99.9 99.9 mass Resin Thermal W/m K 27 22 23 22 23 23 25 20 8
15 20 sheet conductivity Specific gravity ratio % 100 100 100 100
100 100 100 100 100 98 98 Degree of -- 15 20 22 15 25 24 20 31 114
26 22 orientation
[0225] It is understood from Table 2 that the hBN powders of
Examples 1 to 7 exhibit a high thermal conductivity while
suppressing the anisotropy because the purity is higher, the
thermal conductivity of the resin sheet is higher, and the degree
of orientation is lower for all the hBN powders of Examples 1 to 7
than for the hBN powders of Comparative Examples 1 to 4.
[0226] It is considered that this is because in any of Examples 1
to 7, the primary particle size is less than 10 .mu.m, which is
smaller than in Comparative Examples 1 to 4, the ratio [L1/d1] is
5.0 or more and 20 or less, the BET specific surface area is small,
as small as less than 10 m.sup.2/g, the peak decrease rate is 10%
or more and less than 40%, so that the primary particles of the hBN
powder contact with one another while they are randomly orientated
to form the dense and strong aggregate, thereby making the
aggregate hard to disintegrate.
[0227] The hBN powders of Examples 1 to 7 comprises such an
aggregate, and therefore it is considered that in the hBN powders
of Examples 1 to 7, the aggregate can maintain a granular shape
without disintegrating in the process of forming a composite with a
resin in molding the resin composition comprising the hBN powder
into the resin sheet, and further, in the obtained resin sheets,
the primary particles of the hBN can maintain the random
orientation, so that high thermal conductive properties can be
exhibited, and the anisotropy can be suppressed.
[0228] This is also understood from the fact that in the hBN powder
of Example 1 shown in FIG. 3, the aggregate comprises the primary
particles having a smaller primary particle size than in the case
of the hBN powder of Comparative Example 2 shown in FIG. 4, and
primary particles are randomly orientated.
* * * * *